The command line is one of the most powerful tools available for use with Ubuntu, and indeed Linux. Knowledge of the commands
associated with it and also how to string them together will make your life with Ubuntu much easier.

This chapter looks at some of the basic commands that you need to know to be productive at the command line. You will find
out how to get to the command line, and also get to grips with some of the commands used to navigate around the file system.
Later on in this book is the Command Line Masterclass (Chapter 30), which explores the subject in more depth. The skills you
learn in this chapter will give you confidence when you’re called upon to work at the command line.

What Is the Command Line?

Hang around Linux users for any length of time and it will not be long before you hear them speak in hushed tones about the
command line or the terminal. Quite rightly too, as the command line offers a unique and powerful way to interact with Linux.
However, for the most part you may never need to access the command line because Ubuntu offers a slew of graphical tools that
enable you to configure most things on your system.

But sometimes things go wrong and you may not have the luxury of a graphical interface to work with. It is in these situations
that a fundamental understanding of the command line and its uses can be a real life saver.

NOTE

In Chapter 3, “Working with Gnome,” you learned about BulletProofX, a project whose goal it is to always provide a fallback
if your X server fails. Under Ubuntu 8.04, this has been further improved, although there will still be some instances where
even BulletProofX won’t save you.

Don’t be tempted to skip over this chapter as irrelevant; rather, work through the chapter and ensure that you are comfortable
with the command line before moving on.

It is tempting to think of the command line as the product of some sort of black and arcane art, and in some ways it can appear
to be extremely difficult and complicated to use. However, perseverance is key and by the end of this chapter you should at
least be comfortable with using the command line and ready to move onto Chapter 30, “Command Line Masterclass.”

More importantly, though, you will be able to make your way around a command line–based system, which you are likely to encounter
if you work within a server environment.

This chapter introduces you to a number of commands, including commands that enable you to do the following tasks:

Perform routine tasks—Logging in and out, using the text console, changing passwords, listing and navigating directories

Implement basic file management—Creating files and folders, copying or moving them around the file system, renaming and ultimately deleting them (if necessary)

Execute basic system management—Shutting down or rebooting, reading man pages, and using text-based tools to edit system configuration files

The information in this chapter is valuable for individual users or system administrators who are new to Linux and are learning
to use the command line for the first time.

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Those of you who have used a computer for many years will probably have come into contact with MS-DOS, in which case being
presented with a black screen will fill you with a sense of nostalgia. Don’t get too comfy; the command line in Linux is far
superior to its distant MS-DOS cousin. Whereas MS-DOS skills are transferable only to other MS-DOS environments, the skills
that you learn at the Linux command line can be transferred easily to other Unix-like operating systems, such as Solaris,
OpenBSD, FreeBSD, and even Mac OS X, which allows you access to the terminal.

User Accounts

One concept you will have to get used to is that of user-based security. By and large, only two types of users will access
the system as actual users. The first type is the regular user, of which you created one when you started Ubuntu for the first
time (see Chapter 1, “Installing Ubuntu”). These users can change anything that is specific to them, such as the wallpaper
on the desktop, their personal preferences, and so on. Note that the emphasis should be on anything that is specific to them,
as it prevents regular users from making system-wide changes that could affect other users.

To make system-wide changes, you need to use super-user privileges which you should have if your account was the first one
specified (i.e. when you specified a user during the installation). With super-user privileges you basically have access to
the entire system and can carry out any task, even destructive ones! In order to use your super-user privileges you need to
prefix the command you wish to execute with the command sudo. When you hit enter (after typing the remaining command) you will be prompted for your password, which you should type in
followed by the Enter key. Ubuntu will then carry out the command, but with super-user privileges.

An example of the destructive nature of working as the super-user can be found in the age-old example of $sudo rm -rf /, which erases all the data on your hard drive. You need to be especially careful when using your super-user privileges, otherwise
you may make irreparable damage to your system.

Don’t let this worry you, though, as the ability to work as the super-user is fundamental to a healthy Linux system. Without
it you would not be able to install new software, edit system configuration files, or do any number of administration tasks.
By the end of this chapter you will feel comfortable working with your super-user privileges and be able to adequately administer
your system.

Ubuntu works slightly differently to other Linux distributions by giving users super-user privileges by default. If you work
with any other Linux distro you will quickly come across the root user, which is a super-user account. So rather than having to type in sudo before every command, the root account can simply issue the command and not have to worry about entering a password. You
can tell when you are working at a root prompt because you will see the pound sign (#). Within Ubuntu the root account is disabled by default in preference to giving super-user privileges to users. If you wish to enable the root account
then issue the command sudo passwd. When prompted, enter your user password. You will then be asked for a new UNIX password; this will be the password for the
root account, so make sure and remember it. You will also be prompted to repeat the password, in case you’ve made any mistakes.
Once you’ve typed it in and pressed Enter, the root account will now be active. You’ll find out how to switch to root later on.

An alternative way of getting a root prompt, without having to enable the root account, is to issue the command sudo -i. After entering your password you will find yourself at a root prompt (#). Do what you need to do and when you are finished, type exit and press Enter to return to your usual prompt.

As with most things, Ubuntu offers you a number of ways to access the command line. You can use the Terminal entry in Applications,
Accessories, but by far the simplest way is to press Ctrl + Alt + F1. Ubuntu switches to a black screen and a traditional
login prompt that resembles the following:

Ubuntu 8.04(Hardy) hardy-dev tty1
hardy-dev login:

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This is actually one of six virtual consoles that Ubuntu provides for your use. After you have accessed a virtual console,
you can use the Alt key and F1 through F6 to switch to a different console. If you want to get back to the graphical interface,
press Alt + F7. You can also switch between consoles by holding the Alt key and pressing either the left or the right cursor
key to move down or up a console, such as tty1 to tty2.

Ubuntu is waiting for you to log in as a user, so go ahead and enter your username and press the return key. Ubuntu then prompts
you for your password, which you should enter. Note that Ubuntu does not show any characters while you are typing your password
in. This is a good thing because it prevents any shoulder surfers from seeing what you’ve typed or the length of the password.

Hitting the Return key drops you to a shell prompt, signified by the dollar sign:

andrew@hardy-dev ~]$

This particular prompt tells me that I am logged in as the user andrew on the system hardy-dev and I am currently in my home directory (Linux uses the tilde as shorthand for the home directory).

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Navigating through the system at the command line can get confusing at times, especially when a directory name occurs in several
different places. Fortunately, Linux includes a simple command that tells you exactly where you are in the file system. It’s
easy to remember because the command is just an abbreviation of present working directory, so type pwd at any point to get the full path of your location. For example, typing pwd after following these instructions shows /home/yourusername, meaning that you are currently in your home directory.

Using the pwd command can save you a lot of frustration when you have changed directory half a dozen times and have lost track.

Another way to quickly access the terminal is to go to Applications, Accessories and choose the Terminal entry. Ubuntu opens
up gnome-terminal, which allows you to access the terminal while remaining in Gnome. This time, the terminal appears as black text on a white background. Accessing the
terminal this way, or by using the Ctrl + Alt + F1 method makes no difference because you are interacting directly with the
terminal itself.

Navigating Through the File System

Use the cd command to navigate through the Ubuntu file system. This command is generally used with a specific directory location or
pathname, like this:

$ cd /etc/apt/

Under Ubuntu, the cd command can also be used with several shortcuts. For example, to quickly move up to the parent (higher-level) directory, use the cd command like this:

$ cd ..

To return to one’s home directory from anywhere in the Linux file system, use the cd command like this:

$ cd

You can also use the $HOME shell environment variable to accomplish the same thing. Type this command and press Enter to return to your home directory:

$ cd $HOME

You can accomplish the same thing by using the tilde (~) like this:

$ cd ~

Don’t forget the pwd command to remind you where you are within the file system!

Another important command to use is the ls command, which lists the contents of the current directory. It’s commonly used by itself, but a number of options (or switches)
available for ls give you more information. For instance, the following command returns a listing of all the files and directories within
the current directory, including any hidden files (denoted by a . prefix) as well as a full listing, so it will include details such as the permissions, owner and group, size and last modified
time and date:

$ ls -al

You can also issue the command

$ ls -R

which scans and lists all the contents of the sub-directories of the current directory. This might be a lot of information,
so you may want to redirect the output to a text file so you can browse through it at your leisure by using the following:

$ ls -alR > listing.txt

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The previous command sends the output of ls -alR to a file called listing.txt, and demonstrates part of the power of the Linux command line. At the command line you are able
to use files as inputs to commands, or generate files as outputs as shown. For more information about combining commands,
see Chapter 30.

We’ve included a table showing some of the top-level directories that are part of a standard Linux distro in Table 4.1.

Table 4.1. Basic Linux Directories

Name

Description

/

The root directory

/bin

Essential commands

/boot

Boot loader files, Linux kernel

/dev

Device files

/etc

System configuration files

/home

User home directories

/initrd

Initial RAM disk boot support (used during boot time)

/lib

Shared libraries, kernel modules

/lost+found

Directory for recovered files (if found after a file system check)

/media

Mount point for removable media, such as DVDs and floppy disks

/mnt

Usual mount point for local, remote file systems

/opt

Add-on software packages

/proc

Kernel information, process control

/root

Super-user (root) home

/sbin

System commands (mostly root only)

/srv

Holds information relating to services that run on your system

/sys

Real-time information on devices used by the kernel

/tmp

Temporary files

/usr

Secondary software file hierarchy

/var

Variable data (such as logs); spooled files

Knowing these directories can aid you in partitioning in any future systems, letting you choose to put certain directories
on their own distinct partition.

Some of the important directories in Table 4.1, such as those containing user and root commands or system configuration files, are discussed in the following sections.
You use and edit files under these directories when you use Ubuntu.

Linux also includes a number of GNU commands you can use to search the file system. These include the following:

whereiscommand—Returns the location of the command and its man page.

whatiscommand—Returns a one-line synopsis from the command’s man page.

locatefile—Returns locations of all matching file(s); an extremely fast method of searching your system because locate searches a database containing an index of all files on your system. However, this database (about 4MB in size and named slocate.db, under the /var/lib/slocate directory) is built daily at 4:20 a.m. by default, and does not contain pathnames to files created during the workday or in the evening. If you do not keep your machine on constantly, you can run the updatedb command either using sudo or by using the root account to manually start the building of the database.

apropossubject—Returns a list of commands related to subject.

Managing Files with the Shell

Managing files in your home directory involves using one or more easily remembered commands. If you have any familiarity with
the now-ancient DOS, you recognize some of these commands (although their names are different from those you remember). Basic
file management operations include paging (reading), moving, renaming, copying, searching, and deleting files and directories.
These commands include the following:

Note that each of these commands can be used with pattern-matching strings known as wildcards or expressions. For example, to delete all files in the current directory beginning with the letters abc, you can use an expression beginning with the first three letters of the desired filenames. An asterisk (*) is then appended to match all these files. Use a command line with the rm command like this:

$ rm abc*

Linux shells recognize many types of filenaming wildcards, but this is different from the capabilities of Linux commands supporting
the use of more complex expressions. You learn more about using wildcards in Chapter 11, “Automating Tasks.”

NOTE

Learn more about using expressions by reading the grep manual pages (man grep).

Working with Compressed Files

Another file management operation is compression and decompression of files, or the creation, listing, and expansion of file
and directory archives. Linux distributions usually include several compression utilities you can use to create, compress,
expand, or list the contents of compressed files and archives. These commands include

bunzip2—Expands a compressed file

bzip2—Compresses or expands files and directories

gunzip—Expands a compressed file

gzip—Compresses or expands files and directories

tar—Creates, expands, or lists the contents of compressed or uncompressed file or directory archives known as tape archives or tarballs

Most of these commands are easy to use. The tar command, however, has a somewhat complex (although capable) set of command-line options and syntax. Even so, you can quickly
learn to use tar by remembering a few simple invocations on the command line. For example, to create a compressed archive of a directory,
use tar’s czf options like this:

$ tar czf dirname.tgz dirname

The result is a compressed archive (a file ending in .tgz) of the specified directory (and all files and directories under it). Add the letter v to the preceding options to view the list of files added during compression and archiving. To list the contents of the compressed
archive, substitute the c option with the letter t, like this:

$ tar tzf archive

Of course, if many files are in the archive, a better invocation (to easily read or scroll through the output) is

$ tar tzf archive | less

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In the previous code example, we used a pipe character (|). Each pipe sends the output of the first command to the next command.
This is another of the benefits of the command line under Linux—you can string several commands together to get the desired
results.

To expand the contents of a compressed archive, use tar’s zxf options, like so:

$ tar zxf archive

The tar utility decompresses the specified archive and extracts the contents in the current directory.

Use Essential Commands from the /bin and /sbin Directories

The /bin directory (about 5MB if you do a full install) contains essential commands used by the system for running and booting Linux.
In general, only the root operator uses the commands in the /sbin directory. Many (though not all) these commands are statically linked which means that such commands do not depend on software libraries residing under the /lib or /usr/lib directories. Nearly all the other applications on your system are dynamically linked—meaning that they require external software libraries (also known as shared libraries) to run.

Use and Edit Files in the /etc Directory

More than 10MB of system configuration files and directories reside under the /etc directory if you install all the software included with this book. Some major software packages, such as Apache, OpenSSH,
and xinetd, have directories of configuration files under /etc. Other important system-related configuration files in /etc are

fstab—The file system table is a text file listing each hard drive, CD-ROM, floppy, or other storage device attached to your PC.
The table indexes each device’s partition information with a place in your Linux file system (directory layout) and lists
other options for each device when used with Linux (see Chapter 32, “Kernel and Module Management”). Nearly all entries in
fstab can be manipulated by root using the mount command.

modprobe.d/—This folder holds all the instructions to load kernel modules that are required as part of the system startup, and replaces
the historic modprobe.conf file.

passwd—The list of users for the system, along with user account information. The contents of this file can be changed by various
programs, such as useradd or chsh.

shells—A list of approved shells (command-line interfaces).

Protect the Contents of User Directories—/home

The most important data on a Linux system resides in the user’s directories, found under the /home directory. Segregating the system and user data can be helpful in preventing data loss and making the process of backing
up easier. For example, having user data reside on a separate file system or mounted from a remote computer on the network
might help shield users from data loss in the event of a system hardware failure.

Use the Contents of the /proc Directory to Interact with the Kernel

The content of the /proc directory is created from memory and exists only while Linux is running. This directory contains special “files” that either
extract information from or send information to the kernel. Many Linux utilities extract information from dynamically created
directories and files under this directory, also known as a virtual file system. For example, the free command obtains its information from a file named meminfo:

The /proc directory can also be used to dynamically alter the behavior of a running Linux kernel by “echoing” numerical values to specific
files under the /proc/sys directory. For example, to “turn on” kernel protection against one type of denial of service (DOS) attack known as SYN flooding, use the echo command to send the number 1 (one) to the following /proc path:

$ sudo echo 1 >/proc/sys/net/ipv4/tcp_syncookies

Other ways to use the /proc directory include

Getting CPU information, such as the family, type, and speed from /proc/cpuinfo.

Viewing important networking information under /proc/net, such as active interfaces information under /proc/net/dev, routing information in /proc/net/route, and network statistics in /proc/net/netstat.

Retrieving file system information.

Reporting media mount point information via USB; for example, the Linux kernel reports what device to use to access files
(such as /dev/sda) if a USB camera or hard drive is detected on the system. You can use the dmesg command to see this information.

Getting the kernel version in /proc/version, performance information such as uptime in /proc/uptime, or other statistics such as CPU load, swap file usage, and processes in /proc/stat.

Work with Shared Data in the /usr Directory

The /usr directory contains software applications, libraries, and other types of shared data for use by anyone on the system. Many
Linux system administrators give /usr its own partition. A number of subdirectories under /usr contain manual pages (/usr/share/man), software package shared files (/usr/share/name_of_package, such as /usr/share/emacs), additional application or software package documentation (/usr/share/doc), and an entire subdirectory tree of locally built and installed software, /usr/local.

Temporary File Storage in the /tmp Directory

As its name implies, the /tmp directory is used for temporary file storage; as you use Linux, various programs create files in this directory.

Access Variable Data Files in the /var Directory

The /var directory contains subdirectories used by various system services for spooling and logging. Many of these variable data files,
such as print spooler queues, are temporary, whereas others, such as system and kernel logs, are renamed and rotated in use.
Incoming electronic mail is usually directed to files under /var/spool/mail.

Linux also uses /var for other important system services. These include the top-most File Transfer Protocol (FTP) directory under /var/ftp (see Chapter 18, “Remote File Serving with FTP”), and the Apache web server’s initial home page directory for the system,
/var/www/html. (See Chapter 17, “Apache Web Server Management,” for more information on using Apache.)